A fluororesin such as polytetrafluoroethylene [PTFE] is excellent in heat resistance, chemical resistance, water and oil repellency, mold releasability and sliding property, among others. Therefore, a fluororesin is used as a coating agent in the field of mold release agents, rolls in office automation [OA] equipments, irons and other household utensils, frying pans, hot plates and other kitchen utensils, food industry, electric industry, machine industry and so forth.
On the other hand, a fluororesin has a problem in that, due to its nonstickiness, they are poor in adhesion to an article to be coated. For improving this adhesion, it has become a common practice to coat an article with a coating composition containing a binder resin such as a heat-resistant resin in combination with a fluororesin.
The following compositions, for example, have been proposed as a coating composition comprising a fluororesin and a heat-resistant resin: a coating composition comprising a fluororesin, mica or metal flake, a depolymerizable acrylic resin and a liquid carrier (cf. Patent Document 1) and a coating composition comprising a fluororesin and a specific polyether urethane resin (cf. Patent Document 2 and Patent Document 3).
The following compositions have also been proposed as such a coating composition improved in coating performance: a coating composition comprising a melt-processable fluororesin showing a melt viscosity in a specific range and an acrylic copolymer showing a decomposition temperature not exceeding 40° C. (cf. Patent Document 4) and a coating composition comprising a fluororesin particle, a high-boiling polyhydric alcohol, a depolymerizable acrylic resin and a nonionic surfactant and an aqueous medium (cf. Patent Document 5).
Generally, these coating compositions can be prepared by adding a resin-containing aqueous dispersion or the like to a fluororesin-containing aqueous dispersion. The fluororesin-containing aqueous dispersion is the one obtained by carrying out polymerization in the presence of an aqueous medium containing a fluorine-containing surfactant; however, the fluorine-containing surfactant is expensive and, therefore, it is preferable that the content thereof in the fluororesin-containing aqueous dispersion be reduced for the purpose of recovering the same, for example.
Methods known for reducing the fluorine-containing surfactant level in a fluororesin-containing aqueous dispersion comprise, for example, concentration by phase separation, electrical concentration and/or ion exchange concentration (cf. e.g. Patent Document 6, Patent Document 7 and Patent Document 8). However, a fluororesin-containing aqueous dispersion having a low fluorine-containing surfactant concentration has such problems as failure to stably dispersing a fluororesin and poor coating performance of a coating composition in which a fluororesin is used.    Patent Document 1: Japanese Kokai (Laid-open) Publication S52-13531    Patent Document 2: International Publication WO 97/40112    Patent Document 3: International Publication WO 99/21927    Patent Document 4: Japanese Kohyo (Laid-open under PCT) Publication H11-513052    Patent Document 5: International Publication WO 2003/011991    Patent Document 6: International Publication WO 2004/050719    Patent Document 7: Japanese Kohyo Publication 2002-532583    Patent Document 8: Japanese Kokai Publication 2006-188704